This invention relates to an artificial knee prosthesis, or a prosthetic knee, for the replacement of a natural knee through surgical implantation.
In general, a natural knee is formed by the two condyles of the bottom part of the femur, the lower surfaces of which bear upon the complementarily shaped upper surface plateaus of the tibia through the intermediary of meniscii, a fibro-cartilage. Connection through the knee is provided by means of ligaments which also provide joint stability and help to absorb stresses applied to the knee. The femur, meniscii and tibia are normally subjected to relatively large forces in the course of supporting the major portion of a person's body weight.
Movement of the normal knee is complex, that is it is not simply a pivotal or rotational movement. Rather, a healty, natural knee has the ability to move in several distinct manners. Primarily, the natural knee joint permits flexion and extension between the femur and the tibia through an arc of about 135.degree.. This motion is a combined rotational, rocking and gliding movement of successive points of the femoral condyles and the tibial plateaus. In addition, the healthy, natural knee permits a relative rotational movement between the condyles and the tibial plateaus as well as some limited relatively sliding motion (in conjunction with other knee motions) which might be described as taking place in a generally horizontal plane between the condyles and the plateau. The knee also permits a rolling-type motion, frequently referred to as abduction and adduction, between the condyles and the plateaus which might best be described as a limited rocking-type motion in a lateral direction, that is generally perpendicular to the plane in which the knee most commonly articulates (flexion and extension).
Aside from the proper geometric configuration of the condyles and the tibial plateaus, an effective, free movement of a natural knee in these directions requires the presence of a fibro-cartilage, commonly referred to as meniscus, between the condyles and the plateaus. When the meniscii becomes damaged, decreased on inflammed, they cease to function properly, the mobility of the knee joint becomes increasingly impaired and movements are accompanied by increasing severe pains.
To alleviate this condition, it is sometimes necessary to replace the natural knee by surgically implanting a prosthetic knee.
The prior art is replete with a great variety of prosthetic knees. In all instances, the prior art seeks to approximate the mobility of a natural knee to a greater or lesser extent by providing artificial substitutes for the condyle surfaces, the condyles as a whole, the entire lower portion of the femur, the tibial plateau or the entire upper portion of the tibia, including the plateau. The following U.S. Pat. Nos. disclose a variety of differently constructed and operating prosthetic knees:
No. 3,715,763 PA0 No. 3,774,244 PA0 No. 3,813,700 PA0 No. 3,824,630 PA0 No. 3,852,830 PA0 No. 3,924,277 PA0 No. 3,964,106 PA0 No. 4,000,525 PA0 No, 4,034,418 PA0 3,760,427 PA0 3,795,922 PA0 3,824,630 PA0 3,837,009 PA0 3,848,276 PA0 3,909,854 PA0 3,918,101 PA0 3,934,272 PA0 3,969,773 PA0 4,001,896
Further, the article "The Surgical Replacement of the Human Knee Joint" by D. A. Sonstegard, L. S. Matthews and H. Kaufer, published in the January 1978 issue (Vol. 238, No. 1) of Scientific American on pages 44-51, gives a good summary of the problems encountered with prosthetic knees and discusses the current state of the art in general terms.
There are two main types of prosthetic knees. The first type is an articulated device which provides a constraining mechanical linkage between the femoral and tibial implants; that is one in which the two implants are mechanically constrained to each other by means of a hinge, a ball and socket type joint, etc.
The second type of prosthetic knee is a non-constrained or unlinked device. In these devices, the emphasis is on replacing some or all of worn and/or deceased load bearing surfaces of the knee. They do not include a mechanical link, hinge, or similar constraint between the femoral and tibial implants. Instead, the stability of the prosthetic knee is primarily provided by the patient's ligaments and muscles.
The present invention relates to knees of the former type, namely constrained or articulated prosthetic knees.
Generally speaking, the replacement of an entire natural knee requires the surgical implantation of the prosthetic knee by removing at least portions of the femoral condyles and the tibial plateaus and implanting therein normally metallic substitutes referred herein as "femoral implants" and "tibial implants".
Although the exact construction of the femoral and tibial implants of such prosthetic knees and the exact manner in which they are connected to the femur and the tibia, respectively, varies, they all have the common characteristic that the implants are secured to the femur and the tibia with a bonding agent or cement.
The two implants of an articulated prosthetic knee are movably, normally pivotally interconnected so that they cannot move axially away from each other. Accordingly, such knees form a mechanical link between the patient's femur and tibia and they can be used when his ligaments are damaged or unusable.
The following is a list of relevant U.S. Pat. Nos. which disclose articulated knee prothesis:
Typically, these patents disclose prosthetic knees in which the relative motions between the femoral and the tibial implants are pivotal motions about a fixed pivot axis oriented transversely to the flexion-extension plane of the knee.
It is quite apparent that an articulated knee which incorporates a fixed pivot axis has a greatly reduced mobility or motion freedom as compared to a natural knee. In particular, such a knee is typically movable in only the primary direction, as above described, namely in the flexion-extension plane of the knee. Moreover, the pivotal articulation of the prosthesis is at best an approximation of the actual motion path of a natural knee, which as above-described, is a complex, arcuate and not a simple, circular path. The difference in the motion paths between a natural knee and a pivoted knee prosthesis causes the additional potential problems of (a) an interference of portions of the patient's femur and tibia which, if it is to be avoided, can severely limit the flexion-extension arc of the prosthesis as compared to that of a natural knee, (b) an adverse affect on the proper functioning of the muscles due to a change in the relative moment arms between the points on which the muscles act on the bones and the pivot axis, and (c) the generation of undesirable forces, bending moments and stresses in the cement interface between the implants and the corresponding bone structures.
It is quite apparent that these factors can adversely affect the operation of pivoted knee prosthesis. In addition, the expected service life might be shortened by them since, generally speaking, implanted knee prosthesis normally fail at the cement interface, and the generation of undesirable stresses often hastens such failures.
Thus, there is presently a need for an improved, pivotally articulated knee which provides a maximum, and ideally a full flexion-extension movement, which does not adversely affect the working of the associated muscles, and which protects the cement interface from stresses which can shorten the service life of the knee and/or result in its premature failure.